The present invention relates generally to optical apertures, and more specifically to an optical window having a dark aperture that may be used in packaging digital mirror display (“DMD's”)-type spatial light modulators (“SLM'S”) and other optical devices.
DMD's are a type of SLM that are used in display technology. DMD's are basically an array of very small mirrors fabricated using photolithographic techniques adapted from semiconductor fabrication. The mirrors, which are commonly referred to as pixels, are modulated with electronic signals to direct light incident on the mirrors in different directions. One DMD has 800 by 600 pixels and consists of an array of 16 by 16 μm tiltable aluminum mirrors mounted on hinges over a complementary metal-oxide-semiconductor (“CMOS”) static random access memory (SRAM) chip, where the mirrors are arranged on a 17-μm pitch to provide a fill factor of nearly 90%. The chip is typically packaged in a package providing the electrical pin-out connections and having a window for coupling light to and from the mirror array.
In many cases, the window is a piece of glass and an aperture is formed defining a clear center portion surrounded by a dark perimeter. The perimeter outlines the active area of the DMD, and traps stray or scattered light in the optical system. In some devices, the dark aperture is formed from alternating thin film layers of chromium oxide (nominally Cr2O3) and chromium formed by either a lift-off or etch method that forms a dark mirror interference stack.
It is desirable that the edge of the aperture is sharply defined, and also that the aperture be aligned with the active pixel area of the DMD. Unfortunately, defining the aperture can leave a relatively ragged edge. This edge can form a bright reflective region at the aperture edge and appear as a thin border around a projected image. In some products, several rows of otherwise good mirrors underlying the aperture edge are intentionally not active, thus consuming excess chip area and reducing the potential yield of chips per wafer because of the ragged aperture edge.
Another problem that can arise is delamination of the thin film stack from the substrate. A chromium layer is often included to promote adhesion of the thin film stack to the glass substrate of the window; however, this chromium layer can limit how dark the thin film stack is. Gross delamination of the dark aperture can occur if the adhesion layer is simply omitted.
Therefore, a window for an optical package that has a dark aperture with fine edge definition and with reduced edge reflection is desirable.